1. Field of the Invention
The present invention relates generally to color-signal converting circuits and, more specifically, to a color-signal converting circuit for generating color display signals for use with a cathode ray tube used in a character and image information display system.
2. Description of the Prior Art
Character and image information systems, such as videotex, teletext, and the like, are known that use memorized conversion tables, for example, a color look-up table in order to display character and image information on cathode ray tube displays with relatively small memories. In conjunction with these systems, there are also known systems in which an independent picture, for example, an animation picture or the like, not requiring a look-up table is displayed on the same display screen in a so-called superposed state with the picture based upon the contents of the look-up table.
In regard to the display of the picture image using the look-up table, in one example of such known system there are produced four-bits of color-display data for each of the red (R), green (G), and blue (B) signal data from the look-up table. The data thus derived are converted to analog signals by a digital-to-analog converter (D/A) and then supplied to a color cathode ray tube for visual display. On the other hand, it is also known to use data for displayaing a color picture image that does not require the use of a look-up table, and which are represented by one bit for each element of the red (R), green (G), blue (B), and brightness or luminance (Y) signal. Therefore, when a picture image derived from a look-up table and a picture image that need not be derived from a look-up table are to be displayed together in the so-called superposed state, in this example it is necessary to convert the one-bit data to four-bit data for each of the red, green, and blue data points and then mix it with the display data derived from the look-up table. Additionally, because the picture image that is not derived from the look-up table can typically have three brightness conditions, such as full brightness, half brightness, and no brightness (dark), the brightness data must also be converted to four-bit display data, such as "1111", "0111", and "0000", respectively, and then also mixed with the display data derived from the look-up table.
Therefore, because both kinds of color-signal data may be presented to this known system, a separate data generating circuit is necessary in order to generate the four-bit display data for each of the red, green, and blue elements, based on each of the original one-bit data points. Also, a mixing circuit is required in order to mix the generated four-bit display data with the four-bit display data derived from the look-up table. This also presents a problem, because two kinds of four-bit color-signal display data are mixed with one another, and because they are generated by different systems, signal processing delays must be taken into consideration and, therefore, the circuit arrangement is necessarily made complicated.
Additionally, the picture derived from the look-up table need not be limited to a single picture and the number may be increased gradually so that many pictures can be displayed in the superposed state. When this becomes a major consideration in known systems, the four-bit signal generator and the mixer circuit and other circuit elements must be increased in scale, and the problems caused by signal processing delays become significant. Also, if display speed is increased then the system timing and delays are also affected so that the entire system arrangement can be adversely affected.